Multiple path parameters in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when sending JSON with two body parameters?

Consider this FastAPI endpoint that expects two body parameters:

from fastapi import FastAPI, Body

app = FastAPI()

@app.post("/items/")
async def create_item(name: str = Body(...), description: str = Body(...)):
    return {"name": name, "description": description}

If you send a POST request with JSON body {"name": "Book", "description": "A nice book"}, what will be the response?

FastAPI

from fastapi import FastAPI, Body

app = FastAPI()

@app.post("/items/")
async def create_item(name: str = Body(...), description: str = Body(...)):
    return {"name": name, "description": description}

A 422 Unprocessable Entity error

B {"name": "Book", "description": "A nice book"}

C {"name": "Book"}

D {"description": "A nice book"}

💡 Hint

Think about how FastAPI extracts multiple body parameters from JSON.

FastAPI can extract multiple parameters from the JSON body if each is declared with Body(...). The JSON keys must match the parameter names. Here, both name and description are required, so sending both returns them in the response.

📝 Syntax

intermediate

Which code correctly defines multiple body parameters in FastAPI?

Choose the correct FastAPI endpoint definition that accepts two required string parameters title and content from the request body.

A async def post_article(title: str = Body(...), content: str = Body(...)):

B async def post_article(title: str, content: str):

C async def post_article(body: dict): title = body['title']; content = body['content']

D async def post_article(title: str = Query(...), content: str = Query(...)):

💡 Hint

Remember that Body(...) is needed to extract parameters from the JSON body.

Option A correctly uses Body(...) to declare that title and content come from the request body. Option A misses Body, so FastAPI treats them as query parameters. Option A is invalid syntax for FastAPI parameters. Option A uses Query which extracts from URL query, not body.

🔧 Debug

advanced

Why does this FastAPI endpoint raise a validation error?

Given this FastAPI endpoint:

from fastapi import FastAPI, Body

app = FastAPI()

@app.post("/users/")
async def create_user(name: str = Body(...), age: int = Body(...)):
    return {"name": name, "age": age}

If you send JSON {"name": "Alice"} only, what error occurs and why?

FastAPI

from fastapi import FastAPI, Body

app = FastAPI()

@app.post("/users/")
async def create_user(name: str = Body(...), age: int = Body(...)):
    return {"name": name, "age": age}

A 400 Bad Request because 'name' is missing

B 200 OK with {'name': 'Alice', 'age': None}

C 422 Unprocessable Entity because 'age' is missing

D 500 Internal Server Error due to missing age

💡 Hint

Check which parameters are required and what happens if one is missing.

Both name and age are required body parameters (due to Body(...)). Missing age causes FastAPI to return a 422 error indicating the request body is incomplete.

❓ state_output

advanced

What is the response when mixing body and query parameters?

Consider this FastAPI endpoint:

from fastapi import FastAPI, Body, Query

app = FastAPI()

@app.post("/products/")
async def add_product(name: str = Body(...), category: str = Query(...)):
    return {"name": name, "category": category}

If you send a POST request with JSON body {"name": "Laptop"} and URL query ?category=electronics, what is the response?

FastAPI

from fastapi import FastAPI, Body, Query

app = FastAPI()

@app.post("/products/")
async def add_product(name: str = Body(...), category: str = Query(...)):
    return {"name": name, "category": category}

A {"name": "Laptop", "category": "electronics"}

B {"name": "Laptop"}

C 422 Unprocessable Entity error

D {"category": "electronics"}

💡 Hint

Recall how FastAPI separates body and query parameters.

The name parameter is extracted from the JSON body, and category is extracted from the URL query string. Both are required, so sending both correctly returns them in the response.

🧠 Conceptual

expert

Why does FastAPI require explicit Body(...) for multiple body parameters?

FastAPI allows only one parameter to be taken from the request body by default. When you want multiple parameters from the body, you must use Body(...) explicitly for each. Why is this design chosen?

A Because <code>Body(...)</code> is required to validate query parameters

B Because FastAPI does not support JSON bodies without <code>Body(...)</code> annotations

C Because Python functions cannot have more than one parameter from the body without <code>Body(...)</code>

D Because HTTP requests can only have one JSON body, so FastAPI needs explicit markers to parse multiple fields separately

💡 Hint

Think about how HTTP request bodies work and how FastAPI maps them to function parameters.

HTTP requests have a single body, usually JSON. FastAPI needs to know which parameters come from that single JSON body. By default, it expects one body parameter. To extract multiple fields separately, Body(...) tells FastAPI to look inside the JSON body for each parameter explicitly.